light management of aluminum doped zinc oxide thin films by fabricating periodic surface textures...

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DOI: 10.1002/adem.201300007
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Light Management of Aluminum Doped Zinc Oxide ThinFilms by Fabricating Periodic Surface Textures Using DirectLaser Interference Patterning**

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By Sebastian Eckhardt, Teja Roch, Christoph Sachse and Andres Fabian Lasagni*
Thin film electrodes for organic photovoltaics and electro-

nics are a major issue at present. For enhanced performance of

organic solar cells and light emitting diodes, an optimized

saturation of light is important. Thin film solar cells offer

several benefits compared to conventional crystalline silicon

modules. They are lightweight, in many cases flexible and can

be produced cost-effectively. Thus they are considered to be a

future technology in photovoltaics (PV). However, one of the

major disadvantages of these cells is a lower efficiency

compared to traditional crystalline solar cells. This lack results

from the lower thickness of the photoactive layer in

combination with a comparatively long absorption length.

In order to achieve a better performance of thin film cells, it is

crucial to increase the interaction probability of the photons by

enhancing their path length within interaction zone of the

photoactive medium. These light management properties can

be achieved by surface patterning of the front back electrode of

the cells with a periodic microstructure presenting diffractive

effects on light like Bragg grids.[1] Using such periodical

surface microtextures, material related characteristics like

transparency, reflection or diffraction can be adjusted. This

characteristics influence the propagation of light within the

cell, so that choosing the right parameters can extend the

optical path of the photons inside the charge separating layer.

[*] Prof. A. F. Lasagni, T. Roch, S. EckhardtInstitute for Surface and Manufacturing Technology, TechnischeUniversitat Dresden, George-Bahr-Str. 3c, 01062 Dresden,GermanyE-mail: [email protected]

C. SachseInstitute for Applied Photo Physics, Technische UniversitatDresden, George-Bahr-Str. 3c, 01062 Dresden, Germany

Prof. A. F. LasagniFraunhofer-Institut fur Werkstoff- und Strahltechnik, Winter-berg Str. 28, 01277 Dresden, Germany

[**] We acknowledge Von Ardenne Anlagentechnik GmbH forproviding the AZO substrates. This work was financiallysupported by the Fraunhofer-Gesellschaft under Grant No.Attract 692174. The financial support of the Allianz IndustrieForschung (AIF, grant Laserstrukturierte Oberflachen furOLEDs und organische Photovoltaik) is also greatly acknowl-edge.

ADVANCED ENGINEERING MATERIALS 2013, 15, No. 10 � 2013 WILEY-VCH V

There are only a few methods of patterning transparent

conducting oxide (TCO) films, with periodic textures. The

most traditional technology is optical interference lithography

on photoresist in combination with surface etching.[2,3] The

interference pattern results from the superposition of two or

more coherent laser beams usually operating in continuous

wave (cw) mode.[2] One common disadvantage of this method

is given by the complexity of the procedure, which requires

more than one production step. As another option, the pattern

can be embossed into the thin film with a stamp.[4,5]

Furthermore, laser writing patterning methods[6,7] can be

used to ablate material from the surface. For this purpose, the

laser beam is scanner-guided over the regions that are to

be ablated. Two more uncommon and newly developed

techniques are nano-molding,[8] where the texture is being

cast in a mould, and micro-molding,[9] which means that

thin films of TCO are laser-deposited on a patterned mold

what transfers the mold’s texture onto the oxide layer as a

result.

The simplest way to obtain large-scale but non-periodic

microstructure on TCO-substrates is etching the surface,[10–13]

which is typically used in modern production processes of

thin film solar cells. These etched surfaces however have

commonly lower diffuse transmission efficiencies compared

to periodical structures.

Differently, the method of Direct Laser Interference

Patterning (DLIP) enables a fast and simple large-scale

fabrication of microscale patterns with spatial periods

normally between 250 nm and 50 mm. Because high energetic

pulsed lasers systems are utilized, this method requires only

one single processing step to generate the surface textures,

avoiding the use of chemicals (e.g., for etching or develop-

ment) or masks. Depending on the laser-type as well as

materials engaged in the procedure, the processing speed

generally varies from 10 to 100 cm2 s�1.[1] A broad variety of

materials such as polymers, metals, semiconductors, and

TCOs have been already patterned using DLIP.[1,6,7,14,15] The

simplest pattern, that can be created with DLIP using a

two-beam set-up is a relief of parallel lines (‘‘line-like

pattern’’). For grid-like patterns, the substrate can be turned

by an angle between 08 and 908 and patterned a second time.

Using three or more beams, more complex patterns can be also

produced.[7,14,15]

erlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 941

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S. Eckhardt et al./Light Management in AZO Thin Film Electrodes

Fig. 1. Experimental setup of direct laser interference patterning with two beams. Theinitial beam which is sent through a cylindrical lens, which leads to a line-like laser spoton the sample, is split by a beam splitter into two partial beams. The beams are laterguided by mirrors to build an overlay-zone on the substrate and a volume of interferinglight waves is formed.

Figure 1 shows a schematic diagram of a 2-beam DLIP

set-up. The principle beam emitted from the laser source

is divided into two equal partial beams applying a beam

splitter. An arrangement of three highly reflective mirrors

guides these two beams to the sample surface, where

they congruently overlap. Because of the coherence of

the electromagnetic waves, interference effects occur in the

superposition zone, which lead to a line-like periodic intensity

distribution given by

IðxÞ ¼ 2I0cos ðkx sin aÞ2; (1)

with I, laser intensity; k, wave vector; x, distance and a, angle

between initial and partial beam. The spatial period L of the

pattern can be calculated using Equation 2:

L ¼ l

2sinðaÞ (2)

with l, wavelength; 2a, angle between the two partial laser

beams.

In this work aluminum doped zinc oxide (AZO) coated

float glass substrates were patterned with line-like and

hexagonal textures using DLIP. AZO is a special type of

TCO that can be deposited in thin layers on a several materials

such as glass and polymers by sputtering processes. The

layers normally consist on an amorphous or fine polycrystal-

line grain structure with high transparency and relative high

electrical conducting, which depends on the doping level of

elements as well as the layer thickness.[16,17] Line patterns

represent the simplest structures that can be fabricated by

laser interference methods and hexagonal patterns have the

largest packing density considering surface textures. Further-

more, hexagonal patterned surfaces provide similar optical

properties as moth eyes, which makes them interesting for

light management systems on thin film optoelectronics.[18]

The parameters for optimal laser processing of the TCO

coatings were determined and the optical (total transmittance,

haze) and surface properties of the processed samples were

942 http://www.aem-journal.com � 2013 WILEY-VCH Verlag GmbH & Co

studied. In addition, the electrical surface properties were also

analyzed.

1. Results and Discussion

1.1. Direct Laser Interference Patterning of AZO Films

In Figure 2, scanning electron microscope (SEM) images of

the untreated as well as line- and hexagonal patterned

AZO-sample surfaces for two different spatial periods are

depicted. The untreated surface topography in Figure 2a is

characterized by small crystals with a sizes ranging from

100 to 200 nm. The film shows a typical polycrystalline

morphology with a columnar structure, and a roughness of

16.8 nm.[19]

In Figure 2b and c show line-like patterns with a sinusoidal

shape. The topography as it can be observed is homogeneous

over the whole structured area. The boundary limits of the

AZO crystals can be noticed in both images. As a result of

the laser patterning process, the fine polycrystalline grain

structure changes to a smooth surface with larger grains

(Figure 2b–e). Furthermore, for substrates irradiated with

relative low laser fluence (275 mJ cm�2), the rough topography

of the initially irradiates surface can be still observed at the

interference minima positions (Figure 2c).

The structuring mechanism of the AZO coated substrates

can be explained as follows. During the laser treatment, the

granular AZO structure is molten at the interference maxima

due to the thermal interaction of the substrate material with

the laser beam. Due to local surface tension gradients induced

by the temperature difference between interference maxima

and minima positions,[14] the molten material moves also from

the hot to the cold regions. The sinusoidal characteristic of the

line-like profile results from the local thermal treatment at

the interference maxima positions, following the shape of the

interference pattern. During the cooling off period, recrys-

tallization of the film takes place obtaining new grains, which

are flatter and larger than the original. For low laser fluences,

less material is molten at the maxima positions which solidify

prior to form a closed structure. Therefore, a part of the

original surface with the columnar structure can still be seen

(Figure 2c).

Additionally, at high laser fluences also small pores

covering some areas of the treated surface could be noticed

(see Figure 2e for hexagonal-like pattern). These artifacts

probably result from the strong ablation (involving both

melting and evaporation) by the high energetic laser pulses.

The structures shown in Figure 2a and d, with a very

smooth and regular topography, and without sharp edges

satisfy all crucial prerequisites for their integration into thin

film solar cells.[1,4,6]

The structure depth and surface topology of the substrates

were measured by atomic force microscopy. Figure 3a–d show

AFM-micrographs and the topography profiles of line-like

and hexagonal patterned AZO-layers. As it can be observed,

lines and pillar-like arrays are formed in a sinusoidal

characteristic, which corresponds to the energy distribution

. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2013, 15, No. 10

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Fig. 2. SEM-micrographs of non-patterned (a), line-like patterned (b and c) and hexagonal patterned (d and e) AZO-films. A laser wavelength of 355 nm was used to fabricate thetextures. The initial columnar microstructure of the non-patterned AZO (a) can also be found at the minima positions of the AZO surface proceed with low laser fluence (b). Smallbubbles and holes resulting from the high energetic pulses can be seen in (b), (d) and (e). Processing parameters: (a) non-patterned (b) 288 mJ cm�2; (c) 275 mJ cm�2; (d) 288 mJ cm�2,300 mJ cm�2.

of the DLIP pattern. The smooth shape of the pattern is

verified by the AFM scans. In Figure 3a inset, also the

grain boundaries or the larger recrystallized grains can be

observed.

Both pattern depth and enlargement of the sample surface

are displayed as functions of energy density in Figure 4. It can

ADVANCED ENGINEERING MATERIALS 2013, 15, No. 10 � 2013 WILEY-VCH Ver

be seen that higher laser fluences correlate with higher

structure depths. In the fabrication process of hexagonal

patterns, the substrates are irradiated twice. Therefore, the

maximum depths observed for this texture are approximately

1.5 times higher than for line-like patterns. The deepest

positions can be observed at the overlapping regions of both

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Fig. 3. Two examples for AFM-micrographs of (a and b) line-like and (c and d) hexagonal patterned AZO-surfaces. The inset in (a) shows a magnified part of the pattern in order togive a better impression of the recrystallized grains.

intensity maxima corresponding to the two utilized laser

pulses for the structuring process. Thus, the highest surface

enlargement is given for the hexagonal patterns and laser

fluences of 300 mJ cm�2. In analogy to the pattern depth, the

aspect ratios (defined as the quotient between structure height

and spatial period) vary from 0.29 to 0.39, whereby the aspect

ratio increases with the laser fluence. Furthermore, the

roughness (Ra) of the patterned surfaces were 55.3 and

63.2 nm for the hexagonal textured sample, for laser fluences

of 275 and 288 mJ cm�2, respectively. It can be stated in

Fig. 4. Pattern depth (a) and surface enlargement (b) of line-like and hexagonal patterned AZat higher laser fluences. Accordingly, the substrate’s surface is enlarged. Wavelength used


general, that the hexagonal textures have a better quality than

the line-like ones concerning the characteristic of the pattern.

This fact can be explained due to the second processing step,

since when the surface is molten for a second time, artifacts

and defects from the first processing step vanish.

1.2. Optical Properties of Patterned AZO Films

By photospectrometric analysis, the total transmittance and

the intensity of the scattered light were measured. The

diffracted part of the total radiation can be described as

O-surfaces as function of laser energy density. For both cases, the structure size increasesfor fabrication: 355 nm.


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Fig. 5. Photospectrometric analysis of hexagonal- and line-like patterned AZO thin films. The graphs show (a and c) the total transmittance and (b and d) the haze of the unstructuredand textured substrates as functions of wavelength. The best transmittance is performed at 300 mJ cm�2 for the line-like and 288 mJ cm�2 for the hexagonal patterned substrate. Thehighest diffraction qualities (higher haze) are shown for substrates irradiated with 288 mJ cm�2.

function of the light wavelength as the Haze (H(l)), defined as

the quotient of diffuse Tdiff and total Tdir transmittance:

HðlÞ ¼ TdiffðlÞTtotðlÞ

(3)

For all measurements, a Shimadzu photospectroscope

UV-3100/MPC-5100 was utilized. Figure 5 shows the total

transmittance and haze for the hexagonal and line-like

patterns and laser fluences between 275 and 300 mJ cm�2.

The oscillating behavior of the transmittance from the

reference substrate can be explained by interference effects

within the thin AZO-layer. The mean value of transparency

for unprocessed AZO is about 80% in an interval between 450

and 800 nm. When material is ablated by DLIP, the thin films

thickness shrinks partially, which involves a slight increase

of transparency. The higher the rate of ablation and the

deeper the pattern, the more light is transmitted through

the AZO-film. This effect can be observed in Figure 5c, where

the plot line of the hexagonal patterned sample exceeds the

mark of 80% considerably. For the line-like texture (Figure 5a)

this effect was also observed in the wavelength range between


450 and 550 nm. The haze calculations in Figure 5b, show a

maximum of 100% near 300 nm for both of the patterns.

Beyond this value, the slope of the two graphs turns

negative, whereby the line-like textures show lower diffrac-

tion qualities compared to the hexagonal texture. For both

pattern geometries, the curve shape levels out for wavelengths

larger than 650 nm. In contrast, the haze of the unstructured

substrate is close to zero in the whole measured range. The

highest transmittance was achieved with a laser fluence of

300 mJ cm�2 for line-like patterns and with 288 mJ cm�2 for

hexagonal textured surfaces (Figure 5a and c). The most

strongly pronounced haze can be found at 288 mJ cm�2 for

both types of pattern (Figure 5b and d).

1.3. Electrical Characteristics

The AZO sheet resistance was measured using a four

point probe, which uses four terminals to sense the ohmic

resistance without the influences by impedance of contact and

wiring. The measuring results are depicted in Table 1. From

these results, it becomes obvious that the electrical resistance

increases with increasing the laser fluence, i.e., increasing


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Table 1. Electrical characteristics of patterned AZO-samples.

Laser fluence[mJ cm�2]

Electrical resistance [V&]

Line pattern Hexagonal pattern

Reference 2.31 2.31

275 2.38 2.41

288 2.41 2.57

300 2.51 2.67

the pattern depth (see Table 1). This performance correlates

with the model of Fuchs and Sondheimer that predicts an

increasing electrical resistance at a shrinking layer thick-

ness.[20,21] However, the resistance measured for all pattern

geometries and laser fluences are close to the initial resistance

(2.31 V&) of the AZO film. Therefore, the reported values for

the structured substrate show that the AZO films are still

usable as electrodes for organic electronics.

2. Conclusions

We have demonstrated that Direct Laser Interference

Patterning can be utilized for surface structuring of aluminum

zinc oxides to improve light management on thin film solar

cells. Line-like and hexagonal textures were fabricated with

laser fluences ranging from 275 to 300 mJ cm�2, utilizing a

pulsed Nd/YAG high power laser system. Using a laser

fluence of 288 mJ cm�2 and one laser pulse, defect free surface

topographies could be obtained. By evaluation of the electrical

and optical properties of the substrates it could be found that

hexagonal patterned surfaces show a higher performance in

both transparency and diffraction properties compared to

line-like textured and non-patterned substrates. Furthermore,

non-significant variations of the electrical resistance of the

structured substrates were observed after the laser treatment.

3. Experimental

Nine hundred nanometers thin films of aluminum doped zinc

oxide (AZO) were magnetron-sputtered on 3 mm thick float glass

substrates (Von Ardenne Anlagentechnik GmbH). Subsequently, the

substrates were cut into squared pieces with an edge length of 2.54 cm.

Ongoing the samples thus obtained were washed with water and

rinsed with ethanol before attaching them to a four-axis positioning

system. After that, the samples were irradiated with one laser pulse

(line-like pattern), respectively two consecutive laser pulses (hex-

agonal pattern) in three groups of different fluences (275, 288, and

300 mJ cm�2). For the experiments the setup depicted in Figure 1 was

utilized.

A pulsed Nd/YAG-laser with a pulse duration of 10 ns, a repetition

rate of 10 Hz and a wavelength of 355 nm was utilized for the

experiments. The principal beam was guided through an iris

diaphragm in order to adjust its diameter. The prepared samples

were patterned with line-like and hexagonal textures. In order to

fabricate hexagonal patterns, the line-textured sample was turned by

608 and processed again in the same way.


The DLIP-processed samples were analyzed with a photo-

spectrometer which is based upon an optical two path system to

measure sample and reference simultaneously. A polarization filter

was used to minimize measurement errors caused by internal

polarization effects of the photospectrometer. Scanning electron

microscope images were realized with a Philips XL-30 ESEM SEG

microscope at a tilted angle of 308. Pattern depth and surface features

were measured by atomic force microscopy using a Jeol JSPM 5200

AFM-device.

Received: January 9, 2013

Final Version: March 26, 2013

Published online: May 21, 2013

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light management of aluminum doped zinc oxide thin films by fabricating periodic surface textures...

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